1. The Field of the Invention
This invention relates generally to systems and techniques for performing wire and cable testing. More specifically, the invention teaches how to utilize the principles of frequency domain reflectometry (FDR) to perform wire and cable testing including determination of wire or cable characteristics such as length, impedance (which is characterized as an open or short circuit condition), the location of an open or short circuit, capacitance, inductance, and resistance, and wherein the frequency domain reflectometry is utilized to perform in-situ cable testing, testing utilizing passive connectivity, and testing for cable fray.
2. Background of the Invention
The benefits of being able to test cables and wires (hereinafter to be referred to as a cable or wire interchangeably) are many. Some reasons are obvious. For example, cables are used in many pieces of equipment that can have catastrophic results if the equipment fails. A good example of this is an airliner. However, the consequences of non-performance do not have to be so dire in order to see that benefits are still to be gained. For example, cables are used in many locations where they are difficult to reach, such as in the infrastructure of buildings and homes. Essentially, in many cases it is simply not practical to remove cable for testing, especially when this action can cause more damage than it prevents.
Given that the need for cable testing is important and in some cases imperative, the question is how to perform accurate testing that is practical, meaning relatively inexpensive and at a reasonable cost. The prior art describes various techniques for performing cable testing. One such technique is time domain reflectometry (TDR). TDR is performed by sending an electrical pulse down a cable, and then receiving a reflected pulse. By analyzing the reflected pulse, it is possible to determine cable length, impedance, and the location of open or short circuits.
One of the main disadvantages of TDR is that the equipment required to perform time analysis of a reflected signal is expensive and often bulky. These factors of cost and size can be critically important. A less costly and bulky system can be used in more places, more often, and can result in great savings in money spent on performing maintenance functions, and by replacing equipment before failure. But more importantly, the greatest benefit may be the saving of lives.
Consider again the airline industry. Miles of cabling inside an airplane is extremely difficult to reach and test. If the cabling is removed for testing, the cabling can be damaged where no damage existed before. Thus, testing can result in more harm than good when cabling must be moved to gain access. But the nature of cable carrying conduit in an airplane simply makes access with bulky testing equipment difficult. In addition, if the electronics for testing cables could remain in-situ, then testing could be automated and used routinely before or after flight, or at any other time that testing was requested. This can be accomplished only with smaller, less expensive testing systems such as can be provided by frequency domain reflectometry.
It is noted that TDR is not the only prior art technique available for cable testing. In standing wave reflectometry (SWR), a signal is transmitted and a reflected signal is received at a directional coupler. The system then measures the magnitude of the reflected signal. A short circuit, an open circuit, and the depth of a null gives the same information as TDR. However, this technique is generally less accurate and nearly as expensive as TDR.
It is worth noting that the prior art sometimes refers to an FDR cable testing system. However, upon closer inspection, the system being described is actually an SWR cable testing system.
The FDR system to be described in this document is capable of very specific determination of cable characteristics. These characteristics include length, impedance (which is characterized as an open or short circuit condition), the location of an open or short circuit, capacitance, inductance, and resistance. However, it would be advantageous to be able to determine these cable characteristics in very specific circumstances, in situations where the testing of the cable must not interfere with operation of a circuit, regardless of the integrity of the testing apparatus, and to be able to determine these characteristics in circumstances that make it difficult to detect a cable fault because of the nature of the fault.
Accordingly, it would be an advantage over the prior art to utilize FDR to perform in-situ testing of cables in cables that can be modified at a connector, to provide “smart wire” capabilities where the FDR cable testing circuits are disposed in the cabling itself, and to perform these tests on a live cable, or one having no signal or power applied.
It would be another advantage over the prior art to provide the FDR cable testing system that operated in a passive mode, such that no interruption in operation of the cable occurred, even if the testing system were to fail.
It would be another advantage over the prior art to utilize the FDR cable testing system that would enable detection of faults that are more difficult to detect than simple open and short circuits, namely cable fraying, wherein the fray may be shorted to ground or open to air.
It would be another advantage to provide a system that would be less costly because of the nature of the FDR cable testing components utilized therein. It would be another advantage to provide a system that is more likely to be used because it is not as difficult to operate as the prior art cable testing equipment, and can be automated in some situations for regular testing even by unskilled personnel.
It is worth noting that the technology being applied to the problem of cable testing by the present invention has not previously been used for this purpose. Specifically, frequency domain reflectometry (FDR) is typically used in radar applications. FDR is based on single frequency radar or stepped frequency radar. It was utilized in a free-space environment where antennas are used to transmit and receive a radar signal. Thus, the results produced when used for cable testing were surprising to those skilled in the art.